The present invention relates generally to the field of fluid seals and more particularly to pressure seal fittings such as low pressure seal fittings used in connection with a nuclear reactor where a low pressure seal is used between the sealing thimble guide tube and thimble tube at the seal table of such a nuclear reactor during refueling and/or repair of the nuclear reactor.
The current invention relates to an alternative sealing device for a low pressure seal which interchanges with a releasably disassembled high pressure compression fitting. The invention has particular usefulness with the refueling of a pressurized water reactor in the commercial nuclear industry.
A pressurized water reactor refueling process occurs in cycles predicated on the reactor load requirements. The refueling process is a routine operation which is normally performed annually. Each refueling cycle requires a series of operations to be performed in preparation for reloading the reactor core. For the sake of economy, it is desirable that the refueling operation be accomplished as quickly as possible. The present invention is generally directed toward simplifying the refueling operations, reducing "downtime" of the nuclear reactor, increasing the reliability of certain components and reducing radioactive waste.
In a typical pressurized water nuclear reactor, the reactor vessel is seated in a concrete well and contains the nuclear reactor core. Pressure fitting arrangements are assembled on an exterior of the reactor well in an area referred to as the seal table. This arrangement permits reactor monitoring instrumentation, known as neutron flux detectors, to access the reactor core through a combination of stainless steel tubes, referred to as thimble guide tubes and thimble tubes.
The thimble guide tube is welded at a first end to the bottom of the reactor vessel where it penetrates the reactor vessel interior. A second end of the thimble guide tube extends through the reactor biological chamber to the seal table, which is a stainless steel plate whose primary function is to provide a seismic support. The second end of the thimble guide tube is welded to the seal table. The thimble guide tube penetrates just beyond the seal table, terminating as a stub. The interior of the thimble guide tube is exposed to the reactor coolant water and provides a primary pressure boundary for the reactor. The thimble guide tube acts as a guide for the thimble tube.
The thimble tube is a relatively smaller hollow stainless steel tube which is housed within the protective boundary of the thimble guide tube. The first end of the thimble tube, which is a closed end or "bullet nose", extends to the top of the reactor core, while a second end, which is open, passes through the guide tube stub to provide an axial passage as a guide path for the neutron flux detector which may travel the entire length of the thimble tube into the reactor core. This is done for monitoring the reactor during operation. The interior of the thimble tube is essentially dry and is maintained at atmospheric pressure. The flux detector is manipulated through the thimble tube to a point abutting the "bullet nose" during the monitoring process. During normal reactor operation, the thimble tubes extend from the upper plate of the core downwardly to the bottom of the vessel where they pass through a penetration in the bottom of the vessel and end at the seal table, outside the vessel well. Normally, the thimble tube is empty. However, during monitoring, a neutron flux detector is pushed through the thimble tube to the top of the core and then retracted. The thimble tube travels through a thimble guide tube into the reactor core.
The thimble guide tube has an interior diameter larger than the exterior diameter of the thimble tube resulting in an annulus, or space, between the two. This necessitates the use of a high pressure fitting to create a barrier to seal the annulus as the thimble tube extends through the thimble guide tube stub opening precluding escape of reactor coolant during reactor operation.
During the refueling process, a high pressure fitting is disassembled to allow the thimble tubes to be retracted a predetermined distance from the reactor core. This is done to facilitate handling of the reactor fuel assemblies for loading and or unloading of the reactor fuel into the reactor core. The reactor is deactivated for the entire refueling process. The water level in the biological chamber is maintained below a reactor vessel flange, which is slightly lower than the high pressure fitting at the seal table, while the thimble tube is being adjusted.
After thimble tubes are retracted from the reactor core and a high pressure seal is disassembled, a pressure seal is temporarily installed to temporarily replace the high pressure seal. The disconnected portion of the high pressure fitting remains attached to the thimble guide tube. A low pressure seal is required for sealing the thimble guide tube vent path to preclude the reactor cavity water from escaping during fuel handling. The reactor cavity is flooded a predetermined depth above the reactor core which enables fuel assemblies to be altered or replaced within the reactor core. It is imperative for the reactor cavity water level to maintain a minimum height of 10 feet above the fuel at all times to effect radiological control in the plant and public at large.
Before the reactor core can be accessed, the reactor head is lifted off the vessel flange and the reactor cavity is flooded approximately 24 feet above the reactor vessel flange. A low pressure seal is installed at the seal table, between the guide tube stub and the thimble tube, to restrain the reactor cavity water prior to reactor head removal.
Safety concerns associated with the low pressure seal are directed toward the seal's ability to provide a fluid seal against the reactor cavity static head water pressure of approximately 14 pounds per square inch (psi) at the seal table during the reactor refueling operation. A recent requirement imposed upon the low pressure seal is that it must maintain a fluid seal during inadvertent transient loads which can occur during reactor maintenance activities while the low pressure seal is installed should inadequate vent paths exist during reactor disassembly and reassembly. These loads can theoretically reach as high as 350 pounds per square inch (psi). This is a requirement that surpasses the low pressure seals taught by the prior art.
Techniques used to form a low pressure seal in the past have several shortcomings. A first shortcoming is that they are time consuming to manipulate because they require multiple steps to complete installation. An example is U.S. Pat. No. 4,728,479, Merkovsky, which teaches a low pressure seal that must fittingly mate with a high pressure seal nut, which is a compression type seal, that employs a compression ring or a ferrule already installed on a guide tube, and is compressed about the guide tube stub to form a fluid seal when mating the male and female compression fitting bodies together. The ferrule also serves as an anchor for the high pressure seal nut to permit the low pressure seal fitting to compress against the low pressure sealing device.
A second shortcoming is that they are unreliable in their ability to maintain a fluid seal of the annulus formed by the guide tube stub inner diameter and the slidable thimble tube outer diameter. Current low pressure seals only expand across the top of the guide stub to the thimble tube, relying on a downward load to compress the seal snugly over the annulus between the guide tube and the thimble tube leaving a small margin for error in installation and in thimble handling. In most instances, this kind of seal is vulnerable to the slightest movement against the thimble tube which causes a breach in the seal. Third, the prior art does not satisfy "shut down risk analysis" pressure ratings now in effect which can range as high as 350 pounds per square inch (psi). And a fourth shortcoming in the prior art is that the low pressure seals have a short life. They must be replaced after each use which adds to the amount of radioactive contaminated waste the nuclear plant facilities must discard.
Over the period of ever-changing modifications, intended to enhance the performance of refueling operations, more attention has been focused on the low pressure seal. The present invention is directed to simplify the refuelling process by providing a manually manipulated low pressure seal while satisfying the safety concerns and reducing radioactive waste by utilizing a reusable seal.